Could gut bacteria play a role in mental health?

If you’ve ever gotten a stomachache from stress, you might suspect that your mental state can affect your digestive system. But, Agent 99, would you believe the relationship could happen in reverse - that your gut bacteria could boss your brain around? A recent piece from The Verge surveys research on this possibility and spotlights the case of a teenage girl whose psychiatrist treated her severe case of obsessive-compulsive disorder and ADHD with probiotics, in addition to psychotherapy and medication. Within a year, author Carrie Arnold reports, the once-hopeless patient was symptom-free.

From the article:

An imbalance in the microbes in Mary’s gut was either contributing to, or causing, her mental symptoms. “The gut is really your second brain,” [James Greenblatt, MD, the patient’s physician] said. “There are more neurons in the GI tract than anywhere else except the brain.”

Other studies referenced in the article look at links between microbes and behavior in mice, and probiotic intake and brain function in healthy women. Arnold summarizes:

Indeed, scientists still aren’t sure exactly which microbial species are part of a healthy microbiome, nor do they know whether certain bacterial strains are absolutely vital to mental functioning, or whether the right balance is what’s key. Furthermore, research still hasn’t parsed which illnesses might be affected by the microbiome and, therefore, treatable using probiotics.

When asked whether our guts could affect our moods, Stanford microbiologist and immunologist Justin Sonnenburg, PhD, who was not involved in the studies discussed in the article, commented:

There's no doubt about it. These microbes are influencing every aspect of our neurobiology. There's a direct connection between the microbes inside our gut and the central nervous system. They're influencing our behavior, our moods, even our decisions. Experiments comparing the behavior of mice that have normal gut-microbe populations with similar mice that have been raised germ-free (and so have no gut microbes) have found huge differences in how docile or aggressive they are, or how aware of their surroundings they are. There are even differences in the degree to which various genes are expressed in brain tissue.